Technical Abstract:
Chronic alcohol abuse is a major risk factor for development of osteoporosis. However, the mechanisms through which chronic alcohol intake induces bone loss remain unclear. Alcohol-induced oxidative stress might be the key event in tissue injury. In this report, we chronically infused EtOH (12g/kg/day) into post-lactational female Sprague-Dawley rats for 4 weeks using total enteral nutrition. We found that EtOH infusion dramatically decreased bone mineral density (BMD) compared to control animals during this period of bone rebuilding following weaning (P < 0.05). EtOH-induced bone loss was completely blocked by administration of the antioxidant N-acetylcysteine (NAC) at 1.2 g/kg/d (P < 0.05). NAC was able to reverse down-regulation of the bone formation markers alkaline phosphatase and osteocalcin in serum and gene expression in bone tissue (P < 0.05). RNA isolated from bone tissue and specific wnt real time array analysis revealed that the majority of wnt signaling components were down-regulated by chronic EtOH infusion (P < 0.05). Real-time PCR confirmed down-regulated gene expression in a subset of the wnt signaling components by EtOH; however, the wnt antagonist DKK1 was up-regulated by EtOH (P < 0.05). Western blot showed that the key canonical wnt signaling molecule beta-catenin protein expression as well as glycogen synthase kinase-3-beta were inhibited by EtOH in bone, and these actions of EtOH were blocked by NAC (P < 0.05). Finally, we transfected TCF/LEF-dependent transcription of a luciferase reporter gene TOPFLASH and beta-catenin plasmid into osteoblastic UMR-106 cells. We found that EtOH trans-inactivated TCF/LEF gene transcription and blocked beta-catenin nuclear translocation in osteoblasts. All of these effects of EtOH in vitro in osteoblasts were also eliminated by NAC pre-treatment. These observations strongly suggest that EtOH can inhibit bone formation through its novel action on suppression of wnt signaling.